In the field of digital communications, data in binary form, that is, logical "1"'s and "0"'s, are represented by two voltage levels, unipolar in nature. The latter signals are commonly referred to as being in an NRZ (non return to zero) format. When these voltage levels are used in confined areas and are not too widely dispersed, transmission of the levels may be accomplished by direct, that is, D.C., coupling. However, the current trend to convert parallel information to serial data streams and to send such information long distances has created the need for amplification of the signals to insure the integrity of the data received.
Signal amplification using D.C. coupling amplifiers is possible, but drift in such amplifiers makes them undesirable. On the other hand, A.C. coupled amplifiers are easier to design and control, but by their very nature, require signals that alternately exhibit phase reversals, that is, have positive-going excursions followed by negative-going ones. Additionally, these alternating patterns must be substantially continuous and free of interruption for any appreciable length of time. Several schemes have been devised for coding the NRZ data to make it compatible with A.C. coupling. These include Manchester coding, frequency shift coding, phase-shift keying, phase-delay keying and various modifications thereof.
The present invention provides a different and unique encoder/decoder for use with A.C. coupled receivers. The design which is implemented with simple logic configurations has the added advantage of allowing a lower bit frequency on a per bit basis or requiring less time to send a corresponding stream of data when compared with the aforementioned encoding/decoding schemes.